1. Field of the Invention
The present invention concerns a method using powder metallurgy for producing molded parts intended to be assembled by self-brazing to metal parts capable of receiving them and said receiving parts.
The present invention also concerns a method for assembling such molded parts to receiving metal parts.
As used in the present document, the term xe2x80x9cself-brazingxe2x80x9d means the autogenous brazing of the molded part on the receiving metal part, the elements effecting brazing being contained in the molded part.
The operation of self-brazing of the molded part on the metal part may comprise or be followed by a diffusion heat treatment in the solid state, thus constituting what is commonly termed a brazing-diffusion operation, which treatment is intended to homogenize the composition and the structure of the molded parts and of the area of connection by self-brazing.
2. Discussion of Background Information
European Patent No. 075,497 teaches a method for assembly by brazing-diffusion of metal parts, such as components made of superalloy for gas turbines, which consists in interposing between the surfaces to be assembled a connecting layer of overall composition corresponding to a superalloy, and in carrying out a brazing-diffusion treatment on this assembly.
The connecting layer consists of an intimate mixture of two alloy powders, the first powder, termed xe2x80x9cbasic powderxe2x80x9d, being a superalloy powder, and the second powder being a brazing powder of Nixe2x80x94Coxe2x80x94Sixe2x80x94B alloy the liquidus temperature of which is below the solidus temperature of the mechanical parts and of the basic powder.
The relatively low melting temperature of the brazing powder is provided by its content of Si and/or of B.
As used in the present document, the term xe2x80x9cmelting elementxe2x80x9d designates an element such as, by way of non-limiting example, silicon or boron, which substantially lowers the solidus temperature of the alloy into which it is introduced.
The brazing-diffusion operation is carried out at a temperature such that the brazing powder melts and, flowing between the basic powder grains, makes it possible to decrease the porosity very rapidly and to obtain a compact connecting layer independently of the volume of powder employed.
The application of the temperature is then maintained to diffuse the melting element or elements B and Si. The homogenization of the composition resulting from such diffusion raises the liquidus temperature of the melted areas which solidify while the temperature is maintained, diffusion continuing in the solid state.
The result at the end of the brazing-diffusion operation is a homogeneous and dense structure with an absence of discontinuity between the parts to be assembled.
European Patent No. 075,497 gives several examples of application to superalloys based on nickel or based on cobalt.
One of the examples concerns the reconditioning of a fixed turbine blade, made of cobalt-based alloy KC25NW according to the AFNOR designation, cracked by thermal fatigue. AFNOR is the French Standards Committee. According to the AFNOR designation, the alloy KC25NW is a cobalt alloy containing approximately 25% chromium based on the weight of cobalt, and lesser amounts of nickel and tungsten.
For that, a paste containing an intimate mixture of the basic powder of Co superalloy, the brazing powder of Nixe2x80x94Coxe2x80x94Sixe2x80x94B alloy and a volatile binding agent is applied in the previously cleaned and/or widened crack.
The binding agent of the type consisting of a solution of acrylic resin in the monomer of the latter is eliminated by pyrolysis during the brazing-diffusion treatment at 1200xc2x0 C.
European Patent No. 075,497 also provides for applying, instead of a paste, a strip or tape obtained by lamination of an intimate mixture of basic and brazing powders and of acrylic resin.
European Patent No. 075,497 also provides for adding on an elementary part of simple shape in the form of a pre-sintered blank obtained from the mixture of basic and brazing powders, the surface of the pre-sintered blank acting as the connecting layer.
The patent cites in particular as application the plugging of hub support holes on hollow turbine blades, made of alloy NK15CADT according to the AFNOR designation, coming from the foundry. According to the AFNOR designation, the alloy NK15CADT is a nickel alloy containing approximately 15% cobalt based on the weight of nickel, and lesser amounts of chromium, aluminum, molybdenum and titanium. For that, there is introduced into the hole a plug consisting of a pre-sintered blank containing 75% by weight of basic powder of alloy NK17CDAT according to the AFNOR designation, and 25% by weight of brazing powder of Nixe2x80x94Coxe2x80x94Sixe2x80x94B alloy, and self-brazing is carried out at 1200xc2x0 C. for 15 minutes. According to the AFNOR designation, the alloy NK17CDAT is a nickel alloy containing approximately 17% cobalt based on the weight of nickel, and lesser amounts of chromium, molybdenum, aluminum, and titanium.
The use of the method described in that patent is nevertheless subject to a certain number of limitations.
The use of pastes or tapes containing, in addition to the basic and brazing powders, a binding agent which it is necessary to decompose and the decomposition products of which must be eliminated during brazing-diffusion, requires the use of furnaces capable of eliminating large quantities of gas coming from the pyrolysis of the binding agent. Such furnaces are poorly suited to the brazing-diffusion operation which takes placed at around 1200xc2x0 C. and generally under vacuum.
Another problem to be solved is the production of elementary parts in the form of pre-sintered blanks. European Patent No. 075,497 does not indicate any method for producing such blanks and envisages only simple shapes.
The shaping of the blanks may be envisaged by uniaxial cold compacting, but this method does not make it possible to produce very slender or thin shapes of homogeneous density because of friction between powder grains or between grains and walls of the compacting die. Moreover, in order to limit the friction, lubricants of the zinc stearate type or similar are used as additive to the powders; these lubricants are capable of introducing zinc into the blank, which element has a harmful effect on the service life of the superalloys.
Other methods for shaping the blanks are in fact used, such as plasma spraying onto a rotating substrate and cutting of tapes by laser.
Plasma spraying makes it possible to produce blanks of tubular shape generated by rotation, by spraying metal powder onto a rotating cylindrical mandrel. Such a method has a very low yield, around 90% of the sprayed powder being sprayed elsewhere than onto the rotating mandrel, which severely affects the cost of producing the blanks, taking into account the extremely high cost of the metal powders sprayed. Furthermore, this method does not ensure clean edges at the end of the blank, which necessitates re-cutting and further increases the cost of the parts.
The laser cutting of tapes obtained by slip casting, elimination of the solvent and sintering, makes it possible only to obtain relatively thin flat parts. The losses resulting from the laser cutting, however, are high and represent ⅔ rds of the material employed.
The consolidation of self-brazing pre-sintered blanks obtained by powder metallurgy is described in U.S. Pat. No. 4,937,042 which concerns the production of fixed facing friction parts of the fins of gas turbines. The blanks consist of a mixture of a first superalloy powder of the Mxe2x80x94Crxe2x80x94Al or Mxe2x80x94Crxe2x80x94Alxe2x80x94Y type not containing Si, and of a second powder of the Mxe2x80x94Crxe2x80x94Alxe2x80x94Si type containing around 10% by weight of Si, the element Si being the melting element and M representing the element Co or the element Ni or a combination of these two elements. The blanks are pre-sintered at a temperature below the solidus temperature of the second powder.
The consolidation brought about by such pre-sintering is very limited, taking into account the low sintering capacity of the superalloys. The blanks thus obtained are therefore not easily handled.
The present invention provides for parts intended to be assembled by self-brazing to receiving metal parts, the molded parts being able to be of very varied and even complex shapes and having relatively precise dimensional characteristics, and being obtained by a powder metallurgy method having a high yield, that is to say, the ratio between the mass of the molded part obtained and the mass of metal powders employed in order to do this is close to 1.
The present invention provides for parts having a controlled relative density close to 1, the density of which is homogenous in the volume of the part and containing no harmful elements or xe2x80x9cpoisonsxe2x80x9d for the wear characteristics of these parts.
The present invention provides for producing relatively slender solid molded parts the length/width ratio of which is, for example, at least 5 and relatively thin hollow molded parts the diameter/thickness ratio of which is, for example at least 10.
The present invention provides a method using powder metallurgy for manufacturing molded parts intended to be assembled by self-brazing to receiving metal parts.
The method employs two metal powders, a first powder termed xe2x80x9cbasic powderxe2x80x9d making it possible to obtain the desired metallurgical characteristics, and a second powder termed xe2x80x9cbrazing powderxe2x80x9d made of alloy including a melting element at a content such that the liquidus temperature of the brazing powder is below the solidus temperature of the basic powder, the basic powder not including in its chemical composition any voluntary addition of melting element.
The chemical composition of the basic powder may if necessary be obtained from a mixture of powders.
The chemical composition of the basic powder and that of the brazing powder make it possible to define a self-brazing temperature which is higher than the liquidus temperature of the brazing powder and below the solidus temperatures of the basic powder and the receiving part.
The method according to the invention comprises the following sequence of steps:
a) a homogenous mixture of basic metal powder, brazing powder and a liquid binding agent is prepared.
As used herein, a xe2x80x9cbinding agentxe2x80x9d is a constituent or a group of constituents making it possible to bind the metal powder grains to one another to form a mixture of homogeneous appearance.
The binding agent of the present invention may comprise additives intended, for example, to facilitate the dispersion, the suspension of the metal powders, or to improve other characteristics of the mixture.
As used herein the xe2x80x9cliquidxe2x80x9d for the binding agent comprises the molten state and corresponds to various consistencies that can be characterized by their viscosity.
The binding agent is selected to be able to gain consistency when it is i.e., assuming a state capable of retaining a shape contrary, for example, to the liquid state.
b) The mixture thus prepared is injected into a mold of the molded; part to be produced, while applying an appropriate pressure to the said mixture.
The geometry of the mold is adapted to that of the molded parts to be produced, taking into account the dimensional variations resulting from the method, which dimensional variations a person skilled in the art is able to predict from experience or take account of from preliminary tests.
The molding is maintained in conditions of temperature, injection pressure and time such that the binding agent gains consistency.
c) Once the binding agent has become consistent, the mold blank is removed from the mould.
d) The binding agent is eliminated from the molded blank by a known appropriate means or combination of means such as, for example, physical, thermal or chemical means. This step is termed xe2x80x9cbinding agent eliminationxe2x80x9d and the blank which emerges from it is termed xe2x80x9cwith binding agent eliminatedxe2x80x9d.
e) The blank with binding agent eliminated is subjected to a sintering treatment intended to densify it to a relative density of at least 95%, sintering being carried out at a temperature higher than the liquidus temperature of the brazing powder but below the temperature of the subsequent self-brazing treatment.
The condition regarding the minimum sintering temperature makes it possible to ensure the sintering in the liquid phase necessary for obtaining molded parts having a relative density close to unity, even in the case of basic powders having a low sintering capacity, a high density after sintering making it possible to limit dimensional changes during self-brazing.
The condition regarding the maximum sintering temperature makes it possible to ensure the formation of a sufficient quantity of liquid phase during self-brazing to assemble the molded part reliably to the receiving part.
The self-brazing conditions themselves are known to a person skilled in the art of this type of assembly.
The sequence of steps of the method of the invention, namely, preparation of an injectable mixture, moulding, extraction, elimination of the binding agent and sintering, corresponds schematically to those of a method for injection moulding of powdery metallic materials, which method is designated by MIM, an abbreviation of xe2x80x9cmetal injection mouldingxe2x80x9d.
Variants of such MIM methods are described for example in U.S. Pat. No. 4,197,118, WO 88/07902 and WO 88/07903.
The MIM techniques described in those patents filed 10 to 20 years ago are used to produce finished parts having a relative density very close to unity, and a person skilled in the art was not taught transpose these techniques to obtain components having characteristics opposed to the characteristics associated with the products resulting from the MIM techniques, the products obtained by the method of the present invention being xe2x80x9csemi-finishedxe2x80x9d, not completely densified and intended to undergo partial fusion when they are made use of.
Advantageously, but not exclusively, the method of the present invention applies to basic metal powders of superalloys based on Ni, Co or Fe. The brazing powder is then an alloy of Ni, Co or Fe in which the melting element is Si, B or both these elements at the same time.
Preferably, the brazing powder contains, in the case where the element Si is used alone or in combination as melting element, 2 to 12% by weight of Si.
Preferably, in the case where the element B is used alone or in combination as melting element, the brazing powder contains 1 to 5% by weight of B.
Preferably, the alloy of the brazing powder is selected from the following list of alloys: Nixe2x80x94Si, Nixe2x80x94B, Nixe2x80x94Coxe2x80x94Si, Nixe2x80x94Coxe2x80x94B, Nixe2x80x94Coxe2x80x94Sixe2x80x94B, Nixe2x80x94Crxe2x80x94Alxe2x80x94Si, Nixe2x80x94Coxe2x80x94Crxe2x80x94Alxe2x80x94Si, Nixe2x80x94Crxe2x80x94B, Nixe2x80x94Coxe2x80x94Crxe2x80x94B.
In the alloy of the brazing powder, the unspecified elements are present in their customary content, taking into account the base materials used and the methods for preparation of the alloy.
Preferably, the percentage by weight of brazing powder in relation to the whole of the two metal powders is between 5 and 40% and depends on the nature of the two powders.
Preferably again, the charge of metal powders is at least 50% by volume in the mixture produced with the binding agent.
As indicated above, different variants of the MIM method may advantageously be employed in the method of the invention.
The mechanism of gain of consistency of the blank in the mold may, according to a first variant of the method of the present invention, be a physical liquid-solid change of state of the binding agent obtained by maintaining the mold at a temperature below the temperature of said change of state.
The temperature of the mould is of course selected such as to produce the solidification of the binding agent in spite of any possible supercooling phenomena and while taking account of the influence of the presence of any additives in the binding agent.
According to a sub-variant of this first variant of the method of the present invention, the binding agent may be or comprise a thermoplastic resin, the binding agent/metal powders mixture then being prepared at a temperature higher than the melting temperature of the binding agent and injected into the mould also at a temperature higher than this melting temperature.
According to another sub-variant of this first variant of the method of the present invention, the binding agent may be an aqueous or non-aqueous system liquid at ambient temperature, and the mixture prepared with the metal powders is injected into a mould cooled to a temperature below the solidification temperature of the binding agent.
The step of elimination of the binding agent in this sub-variant comprises an operation of lyophilization or sublimation of the binding agent.
According to another variant of the method of the present invention, the binding agent is a thermosetting resin and the mechanism of gain of consistency of the binding agent is an accelerated polymerisation of the resin, for example in a heated mould.
According to yet another variant of the method of the present invention, the binding agent is capable of a sol-gel reaction which is employed during the moulding step. The step of elimination of the binding agent then comprises an operation of putting the binding agent, or its essential constituents, back into solution.
Advantageously, in these different variants of the method of the present invention, the step of elimination of the binding agent may comprise an operation of putting into solution at least one component for the binding agent by the chemical action of a solvent for the component or components.
When the binding agent comprises a polymer, the step of elimination of the binding agent may advantageously comprise an operation of depolymerisation of said polymer by the chemical and/or catalytic action of a specific agent.
Again advantageously, the step of elimination of the binding agent may comprise more than one operation, the final operation being an operation of thermal elimination of the binding agent.
Very advantageously in this case, the operation of thermal elimination of the binding agent continues up to a temperature ensuring the start of consolidation or xe2x80x9cpre-sinteringxe2x80x9d of the metal powders. This pre-sintering makes it possible to handle the blanks with binding agent eliminated, without risk of breakage, before subjecting them to the sintering step intended to densify them.
As used herein, the term xe2x80x9csinteringxe2x80x9d is reserved for the operation which transforms the xe2x80x9cblanksxe2x80x9d with binding agent eliminated into low porosity self-brazing molded parts knowing that, during self-brazing, the physical process of sintering and elimination of the residual porosity continues in the molded parts.
Preferably, in order to permit the pre-sintering of the blanks with binding agent eliminated, the thermal elimination of the binding agent is terminated at a temperature within the melting range of the brazing powder and very preferably in the lower half of the range.
Still more preferably, the sintering step is carried out at a temperature approximately 50xc2x0 C. below that of the subsequent self-brazing operation.
Optionally, the operation of thermal elimination of the binding agent and the sintering step may be carried out successively in the same furnace without a return to ambient temperature between these two operations or steps.
The molded parts obtained after sintering have very regular dimensions which require little or no dimensional retouching by machining to be able to fit the receiving parts and produce a solid self-brazed assembly.
Owing to the forced elimination of the binding agent during binding agent elimination, the molded parts resulting from the method of the present invention do not contain any chemical element other than those which constitute the metal powders employed.
The present invention also covers a method of assembly of the self-brazing molded parts, obtained by the manufacturing method according to the present invention, to receiving parts which are superalloy components of aeronautic or land gas turbines.
According to this method of assembly, the alloy of which the basic powder consists is selected for its compatibility with the superalloy of the receiving parts, and the molded part is pre-assembled to the receiving part by arranging it in contact with or at a small clearance from the receiving part. This may impose known conditions regarding the shape and dimensions of the molded part and of the facing portions of the receiving part.
The pre-assembly between these two parts is then brought to a temperature higher than the liquidus temperature of the brazing powder and below the solidus temperature of the basic powder and of the receiving part in order to carry out self-brazing.
Preferably, and in particular during the application of the method to the repair of parts, the self-brazing treatment is followed directly, or after returning to ambient temperature, by a diffusion treatment intended to diffuse the chemical elements, and especially the melting element or elements, and to homogenize the structure of the repaired area.